U.S. Pat. Nos. 5,542,890 and 5,803,861 disclose a toroidal transmission, and U.S. Pat. No. 5,820,514 discloses a V-belt transmission, as continuously variable transmissions for a vehicle wherein a speed ratio is continuously varied. U.S. Pat. No. 4,916,982 also discloses criteria of speed ratio control wherein the speed ratio of these continuously variable transmissions is increased together with the throttle opening of an engine. Herein, speed ratio is defined as the ratio of the rotation speed of the input shaft/rotation speed of the output shaft. Therefore, a larger speed ratio means the rotation speed of the input shaft is higher for the same output shaft rotation speed.